1. Field of the Invention
This invention relates generally to a gyrocompass, and more particularly to a gyrocompass which is provided with means for damping the north-seeking action of a gyro spin axis.
2. Description of the Prior Art
For damping the north-seeking action of a gyro spin axis, known Anshuetz and Plath gyrocompasses are provided with a damping device, commonly referred to as a damping bottle, in a gyro case enclosing a gyro therein. No description will be given of the principle of this conventional damping device but it was already known about in 1910 and a similar method was employed in a prior Brown gyrocompass. The feature of this method is that torque produced by the damping bottle for damping the north-seeking action of the gyro spin axis is applied about a horizontal axis of the gyro (hence, this system will hereinafter be referred to as a horizontal axis damping system) and that the torque is advanced in phase as compared with the tilt of the gyro spin axis relative to a horizontal plane. The compass employing the damping bottle has an advantage that no latitude error is caused when settling relative to the earth pointing to the north but has a disadvantage that the inclination of the gyro spin axis to the horizontal plane is large and flowing of a liquid contained in the damping bottle requires much time, and hence the settling time is relatively long (about 5 hours).
Further, warships usually adopt a method of cutting out the damping device to avoid an acceleration error in the case of turning but, in this case, it is necessary to stop flowing of the liquid in the damping bottle. To this end, the conventional damping bottle method employs an electromagnetic value in the gyro case, and hence is inevitably expensive and complicated in construction.
While, Sperry gyrocompasses employ a known easterly eccentric axis or damping weight as the damping device for the north-seeking action of the gyro spin axis. Each of these systems is of the type that the torque for damping is applied about a vertical axis of the gyro (and hence it will hereinafter be referred to as a vertical axis damping system). With this prior system, the torque is proportional to the inclination angle of the gyro spin axis from the horizontal plane and the inclination of the gyro at the settling time is far smaller than that in the damping bottle system and no liquid is contained in the damping device, so that the settling time is relatively short (3 to 3.5 hours). However, this system has defects such as a latitude error, a fixed error resulting from a north-south mass unbalance torque of the gyro about its horizontal axis, an error in temperature dependency due to a change in the north-south mass unbalance caused by a temperature change and so on. Further, this system has a drawback such that the gyro spin axis causes a drift in the case of cutting out the damping device.
With the use of a known integrator in combination with a damping weight, the spin axis is held horizontal at the time of settling, and the latitude error and the fixed error resulting from the north-south mass unbalance of the gyro can be avoided. However, since there are the possibility that motion of large time constant still remains according to initial conditions of the integrator, the settling time is surely shorter than that of the damping bottle system but longer than that of the usual vertical axis damping system.